1. Field of the Invention
The invention relates to an adsorption/desorption method for the selective separation of diiodonaphthalene isomers.
2. Discussion of the Background
It is well known that isomers of hydrocarbons can be separated by the use of certain adsorbent/desorbent systems. For example, U.S. Pat. No. 3,663,638 discloses the use of zeolite adsorbents for the separation of xylene isomers.
The fundamental operations which occur during the adsorption/desorption process can be described as follows.
(1) In the adsorption step, a feed solution containing a mixture of hydrocarbon isomers is contacted with an adsorbent which selectively adsorbs one or more of the isomers. The unadsorbed or raffinate phase produced by the initial adsorption step is enriched in the isomers which are not adsorbed. The raffinate phase may be concentrated to obtain these less adsorbed species.
(2) In the desorption step, the selectively adsorbed component or components are displaced from the adsorbent by washing the absorbent with a desorbent and recovering the desorbed isomers in the extract stream. The adsorbent is then ready for another cycle of adsorption.
The adsorbent used in the adsorption/separation process must have the ability to selectively adsorb one or more components relative to the remaining components in the mixture. On the other hand, the adsorbent must be able to release the adsorbed components in the presence of a desorbent during the desorption step. Additionally, since the adsorbent will be recycled, the desorbent must be able to be displaced during the following adsorption step by the selectively adsorbed component in the feed mixture to allow the continuous use of the adsorbent in a cyclic manner.
In other words, the adsorbent must more strongly adsorb one component from the mixture relative to the other possible isomers in the feed mixture. The desorbent must be capable of displacing the more selectively adsorbed component and yet be displaced itself by the next adsorption process. Consequently, in the adsorption step there is a competitive adsorption between the more selectively adsorbed component and the desorbent.
These relationships between the adsorbent, desorbent and adsorbed species dictate that a specific adsorbent/desorbent pair be chosen for any particular mixture of isomers which are desired to be separated. This choice is governed by the specific type of adsorbent, chemical structure and properties of the isomers to be separated and properties of the desorbing solvents themselves.
U.S. Pat. No. 2,958,708 discloses the separation of brominated and chlorinated aromatic compounds using adsorbents which are crystalline zeolites. The preferred desorbent is chlorobenzene. U.S. Pat. No. 4,254,062 teaches the use of X or Y type zeolites to separate isomeric di-chlorotoluenes. 2,6-; 2,5-; and 2,4-dichlorotoluenes can be separated from a single mixture using a hydrocarbon desorbent or auxiliary such as decalin, benzene or toluene. U.S. Pat. No. 4,605,799 discloses the separation of halogenated toluene isomers using a Y-type zeolite as the adsorbent and an alkyl aromatic hydrocarbon, 3,4-dihalotoluene or 4-halo-orthoxylene as the desorbent.
The adsorption/desorption processes of the prior art are useful for separating chloro and bromo derivatives of aromatic compounds. Processes which are useful for the separation of iodoaromatic derivatives, and particularly diiodonaphthalenes have not been described.
Accordingly, a need exists for an adsorption/desorption process for separating diiodonaphthalenes economically and on a commercial scale.